The newly identified lymphotropic human herpesvirus-6 (HHV-6) is the etiological agent of roseola in children and emerging evidences suggest HHV-6's involvement in fatal hepatitis, acute and chronic hepatitis, persistent lymphadenopathy, acute mononucleosis, pneumonitis, fatal hemophagocytic syndrome and encephalitis. HHV-6 isolates segregate into two groups by their in vitro growth properties, by DNA restriction analyzes and by their reactivities with our HHV-6 monoclonal antibodies [MAbs]. These two groups are tentatively designated as group A and group B. Our studies with sera from healthy adults suggest that individuals may be infected by one or both groups. As yet, isolates cannot be grouped by their disease association. Our preliminary studies show that these two groups are antigenically closely related, yet distinct viruses. Our overall objectives are to define the antigenic cross-reactivity among the two groups of HHV-6 and to develop reagents for an accurate measurement of infection by the two groups. MAbs and human sera reactivities suggest group specific epitopes in two immunogenic glycoproteins, gp82-gp105 of group A and gpl85-gp2l0 of group B. MAbs against gp82-gpl05 neutralized only group A viruses. Our specific aims of the proposal are: 1. To determine the immunochemical properties of glycoproteins gp82-gpl05 of group A HHV-6 and gpl85-gp2lO of group B HHV-6, to identify and characterize the potential counterparts in the other group, to determine whether these glycoproteins possess exclusively and/or predominantly group specific epitopes and to test the interaction of antibodies against one group with virus and virus infected cells of another group. 2. To identify and sequence the genes coding for these variant glycoproteins and to determine the genetic basis for the variations among the gp82-gplO5 of group A HHV-6 and gpl85-gp2l0 of group B HHV-6. 3. To use MAbs and affinity purified glycoproteins and proteins expressed from cloned genes to evaluate the feasibility of developing assays for the serological differentiation of infection by the two groups of HHV-6. The studies proposed here are significant as they will generate important information about the antigenic cross-reactivity among the HHV-6 groups and an assessment of assays to distinguish infection by the two groups of HHV-6, all of which are essential for a better understanding of HHV-6 epidemiology, association with human diseases and for the eventual development of control measures.